The transdermal route of parenteral drug delivery provides many advantages. Unfortunately, however, many drugs which appear to be ideal candidates for transdermal delivery have a tendency to cause undesirable skin reactions, conditions known as contact sensitivity or contact allergy. Therefore, despite the development of the transdermal drug delivery art, there remains a continuing need for an improved method of overcoming contact sensitization caused by transdermal delivery of a sensitizing drug.
Sensitization is a two-phase process involving distinct biological mechanisms of the human immune system. The first phase is called the induction phase. Induction occurs when the skin of an individual is first exposed to the sensitizing drug. In this phase, the sensitizing drug or antigen is presented to the T lymphocytes (T cells) by the Langerhans cells of the epidermis, either in situ or in the draining lymph node. As a consequence, T cells which recognize the antigen proliferate and to some extent differentiate. Generally, no visible skin reaction is noted during the induction phase. Following induction, some of the individual's lymphocytes are specifically sensitized to the drug.
The second phase of sensitization is called elicitation. Elicitation occurs when the individual is subsequently (i.e., after induction) exposed to the same sensitizing drug. Elicitation causes a skin reaction to occur. The skin reaction occurring during elicitation is known as contact dermatitis. During elicitation, the antigen is once again presented mainly on the Langerhans cells. The T cells, which have proliferated upon prior exposure to the drug (i.e., during the induction phase), now come to the treated site and initiate events which result in local inflammation or contact dermatitis.
Irritation, on the other hand, is a completely different phenomenon from contact (i.e., skin) sensitization. Skin irritation can be caused by a variety of factors including, but not limited to, physical factors (e.g., chafing or occluding the skin in an airtight manner), exposure to certain chemicals, exposure to pH outside the normal pH range of the skin or mucosa, and bacterial overgrowth. Generally, tissue irritation is the manifested result of damage or toxicity to cells in the skin or mucosa caused by their response to a cytotoxic (i.e., irritating) agent. Sensitization, on the other hand, is the result of a response by the immune system to an agent (i.e., an antigen) which is not necessarily irritating.
In general, once the skin has become sensitized, skin reactions occurring after re-exposure to the sensitizing agent are difficult to prevent. For this reason, this invention is directed towards preventing sensitization from occurring, as well as reducing or eliminating pain and discomfort occurring during the elicitation phase after sensitization has already been induced.
It is generally accepted that recognition of an antigen by a T cell during either the induction or the elicitation phase requires that the antigen be associated with a particular molecule (a "class II MHC molecule") on the surface of an antigen presenting cell (APC). This process is termed "antigen presentation". Typical APCs are macrophages and, in the epidermis, Langerhans cells (Friedmann, Curr. Opin. Immunol., 1989, 1:690-693; Aiba et al., Clin. Res., 1990, 38:283A). For presentation to occur, the antigen must be converted to an appropriate form for association with the MHC. Events that lead to the association of an antigen with the cell surface of a class II MHC molecule are collectively referred to as "antigen processing". Processing involves the uptake by an APC of an antigen into acidic intracellular vesicles such as the lysosomes where it is exposed to proteases so that the antigen, if it is a large proteintype molecule, is physically or chemically altered (Ziegler et al., Proc. Natl. Acad. Sci. USA, 1982, 79:175-178). Class II MHC molecules then associate with the antigenic moiety, intracellularly, whereupon the complex is transported to the surface membrane of the APC. Only then will the antigen be effectively recognized by the T cells.
The low pH of intracellular vesicles has been shown to be a factor in regulating the formation of functional antigen/class II MHC complexes. Ziegler et al. (ibid.) have found that by using lysosomotropic agents such as chloroquine or ammonia to increase the lysosomal pH, the activity of the proteases in the lysosomes is decreased, which proteases appear to interact with at least certain antigenic moieties during processing. It has also been found that the binding of antigen to class II MHC molecules is slow at neutral pH but is accelerated and enhanced in an acidic environment (Jensen, J. Exp. Med., 1990, 171:1779-1784). In addition, low lysosomal pH is a factor in regulating the recycling of receptor molecules in the vesicles, so that when the pH is raised, the rate of recycling is reduced (Tietze et al., Biochen. Biophys. Res. Commun., 1980, 93:1-8; Mellman et al., Ann. Rev. Biochem., 1986, 55:663-700; Joshi et al., Cell Immunol., 1990, 125:518-525). This reduction of recycling could reduce the contact between class II MHC molecules and antigen, resulting in fewer complexes being formed.
Lysosomes are small membrane-enclosed organelles which are found within almost all animal cells. Under normal conditions, lysosomes have an internal pH in the range of 4.5 to 5. In contrast, the physiological pH outside the cell is about 7.0. This difference can result in extensive accumulation within the lysosomes of weak bases. The weak bases can permeate the cell and the lysosomal membranes in their uncharged molecular form. However, the low internal pH of lysosomes favors protonation of the weak base molecules; once they are charged, the molecules are relatively membrane-impermeable and less able to pass back through the membrane. Such accumulation of weak bases will raise the pH in the lysosome (Maxfield, J. Cell Biol., 1982, 95:676-681; Ohkuma et al., Proc. Natl. Acad. Sci. USA, 1978, 75:3327-3331).
Other compounds, the ionophores, have also been shown to raise the pH in lysosomes (Maxfield, ibid.; Ohkuma et al., ibid.). The ionophores incorporate in the lysosomal membranes and facilitate the exchange of ions, thereby destroying the normally-existing pH gradient (Pressman, "Alkali Metal Chelators - The Ionophores", in, Eichhorn, ed., Inorganic Biochemistry, Vol. 1, pp. 218-221, Elsevier Scientific Publishing Co., N.Y., 1973).